It has been a common practice to improve the riding comfort of automobiles and other vehicles by attaching a variety of vibration insulators to the source of vibration and noise, thereby reducing vibration and noise entering the compartment. For example, rubber vibration insulators are used as constituents of the torsional damper and engine mount in the case where the engine is the major source of vibration and noise. The resulting effect is absorption of vibration (that originates from the running engine) and reduction of vibration and noise (that enters the compartment and diffuses into the surrounding environment).
The rubber vibration insulator as mentioned above needs such fundamental properties as strength to support heavy objects like engines and ability to absorb and suppress vibration. It also needs heat aging resistance when it is used in a hot environment such as engine room. There is another requirement for damping properties in a cold environment because automobiles distributed recently throughout the global market are often used under harsh conditions in very cold high-latitude regions.
A conventional way of improving damping properties while keeping the fundamental properties (such as tensile elongation and tensile strength) of rubber vibration insulators is by combination of natural rubber or isoprene rubber with butadiene rubber with a high content of cis-1,4, bonds (which is referred to as high-cis butadiene rubber hereinafter). However, the high-cis butadiene rubber is liable to crystallize and become hard in a cold environment, and consequently it does not fully exhibit its damping properties when the engine is started at an extremely low temperature. Although it is possible to avoid crystallization and prevent aggravation of low-temperature properties by using a butadiene rubber with a low content of cis-1,4, bonds and a high content of trans-1,4, bonds (which is referred to as low-cis butadiene rubber hereinafter), the resulting rubber vibration insulator is poor in damping properties and has a high dynamic-to-static modulus ratio.
On the other hand, rubber vibration insulators can be improved in heat resistance if its sulfur content is reduced. However, the reduction of sulfur content to such an extent as to achieve the desired heat resistance is liable to aggravate damping properties and low-temperature performance. A method for improving heat resistance by combining a small amount of sulfur with a bismaleimide compound has been proposed in JP-A H03-258840. Nevertheless, there still is a demand for further improvement as far as damping properties and low-temperature performance are concerned. The proposed method has a disadvantage that the bismaleimide compound deteriorates the fundamental properties if it is added in an excess amount.
There is also a method of improving (or reducing) the dynamic-to-static modulus ratio of the rubber vibration insulator by incorporating diene rubber with a butadiene polymer (containing cis-1,4, bonds in an amount no less than 98.0%) and carbon black of large particle diameter, as proposed in JP-A 2009-298880. However, there is a need for improvement because the low-temperature performance does not reach a demand level.
As mentioned above, the rubber vibration insulator has fundamental properties, damping properties, heat resistance, and low-temperature performance which are mutually contradictory, and it has been difficult to improve such properties altogether by compounding in a conventional way. Thus, there is a demand for a new method of tackling the above-mentioned problems.